Why do they happen?

Direct Sunlight

In this demonstration It is Summer in the Southern Hemisphere and Winter in the Northern Hemisphere. The amount of direct sunlight on one area determines the season. In this drawing the rays are shooting directly at the southern hemisphere and not as much in the Northern Hemisphere; this causes it to be warmer in the Southern Hemisphere like summer and colder in the Northern Hemisphere like winter.

Earth's position around the sun

This drawing shows all the seasons for the Northern and Southern Hemisphere.

1= Northern Hemisphere- Summer. Southern Hemisphere- Winter.

2= Northern Hemisphere- Fall. Southern Hemisphere- Spring

3=Northern Hemisphere- Winter. Southern Hemisphere - Summer

Earth's tilt is 23.5%

Solsitices and Equinoxes

There are also solstice and equinoxes that mark the start of each season. For the Northern Hemisphere the winter solstice occurs on December 21st and is the shortest day of the year, it is in position 3 in the drawing. The summer solstice occurs on June 21st and is the longest day of the year, it is in position 1. The equinoxes are days with equal daytime and nighttime. The autumnal equinox is on September 21st and is in position 2 in the drawing. The Vernal equinox is on March 21st and is in position 4.